1. Field of the Invention
The present invention relates to a method for stripping non-superconductive Ag-based sheath formed surrounding a superconducting tape or wire. In particular, it relates to a stripping method including placing Bi- or Pb-based metal materials on the Ag-based sheath, heating the Bi- or pb-based metal material to 250.degree. C.-450.degree. C. to dissolve the Ag-based sheath, and removing the dissolved Ag-based sheath.
2. Description of the Prior Art
High temperature superconductors are ceramic oxides that are directly drawn into wires or compressed into tapes with difficulty, because they lack the ductileness that metals have, and the tacky coefficient which quartzes have at a temperature higher than 1000.degree. C. Accordingly, in the fabrication of high temperature superconducting tapes or wires, superconducting ceramic oxides are first filled in a tube, drawn into wires or compressed into tapes, and then subjected to sintering (the so-called powder-in-tube technique). The tube for this purpose is usually a silver or a silver alloy, for example a Ag--Cu alloy. In the applications of superconducting tapes or wires, very often it is necessary to joint two or more superconducting tapes or wires together. Conventional jointing methods include solid-state diffusion, brazing, soldering and welding. The jointing designs used include butt joints and lap joints. In view of the mechanical and electrical properties of the jointed superconducting tapes and wires, lap joints are generally utilized. However, both methods require stripping the outer Ag or Ag alloy tube (sheath).
Mechanical cutting is a method for stripping the sheath. However, this method often results in cracking of the inside ceramic superconducting materials, and thus causes a worsening of the properties of the superconducting materials (J. E. Thaczyk, R. H. Arendt, P. J. Garbauskas, B. A. Jones, R. J. Kilmer and K. W. Lay, "Superconducting joints formed between powder-in-tube Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z /Ag tapes", IEEE Transactions on Applied Superconductivity, 3 (1993) 946). Other methods, for example dissolving with mercury and the utilization of electrolytic polishing, are also been used to strip the sheath (Y. K. Tao, Y. Q. Wang, Y. Y. Sue, P. H. Hor and C. W. Chu, "Preparation and characterization of interface morphologies of silver-sheathed superconducting Bi--Sr--Ca--Cu--Oxide ribbons", J. Material Science Letters, 12(1993) 706 and George F. Vandaer Voort: Metallograph Principles and Practice, McGraw-Hill Book Company, Central Book Company, Taipei, Taiwan, 1984, Chap 2, p119-126 and Appendix H, P 591). However, these two methods are also unsatisfactory because mercury volatilizes easily and its vapor are toxic, and electrolytic polishing usually uses toxic cyanides as electrolytic polishing liquid. Another disadvantage of mechanical cutting and electrolytic polishing is that they are only suitable for stripping single-core superconducting tapes or wires.